This invention relates to a photographic element for the production of lithographic printing plates by the silver complex diffusion transfer process.
The lithographic printing plate consists of greasy ink receptive oleophilic image portions and ink repellent oleophobic non-image portions, the latter being generally water receptive hydrophilic areas.
Accordingly, the customary lithographic printing is carried out by feeding both water and colored ink to the printing plate surface to allow the image portions to receive preferentially the colored ink and the non-image portions preferentially water and then transferring the ink deposited on image portions onto a substrate such as paper for example. In order to obtain a print of good quality, therefore, it is necessary that oleophilic activity of the image portions and hydrophilic activity of the non-image portions are both strong enough so that when water and ink are applied the image portions may receive sufficient amount of ink while the non-image portions may completely repel the ink.
There has already been a printing plate in actual use, which is produced by providing a pattern in metallic silver on the plate surface by utilizing the silver diffusion transfer technique and thereafter making the pattern oleophilic or ink receptive. One of the examples is an image receiving sheet or photosensitive material proposed by the present applicants in U.S. Pat. No. 3,728,114, which comprises either physical development nuclei of heavy metals or sulfides thereof alone or a combination of said nuclei with a specific proportion of a polymeric compound as binder. As previously mentioned, an ideal lithographic printing plate is such that the image portion adequately receives an ink while the non-image portion perfectly repels the ink so that no toning on printing may occur. For this reason, various methods for rendering the non-image portion insensitive to oil have been studied according to the type of printing plate material and the printing process.
As known well in printing business circles, the toning in lithography does not originate from single source, but is generally associated with a combination of factors such as, for example, intrinsic properties of the printing plate; qualities of the printing ink, damping water and printing paper; condition of the printing press; and environmental factors such as temperature and humidity of the printing room during printing. Although it is important to carry out printing under printing conditions and environmental conditions maintained at optimal levels, yet there exists a demand from the practical standpoint for printing plates which can be used under conditions in the range as broad as possible.
In the case of the element fundamentally disclosed in the said U.S. Pat. No. 3,728,114, circumstances are similar to those mentioned above and the element has disadvantages in that tolerance limits are narrow for various commercial inks; printing characteristics vary with the lapse of time before or after the plate making; and toning is liable to occur in long run printing. Therefore, improvement of these disadvantages was wanted.